The present invention relates to an image forming apparatus such as a composite-type digital copier that performs image-copying function, facsimile function, printer function and the like, and also relates to an exposure scanning apparatus for use in the image forming apparatus.
With a conventional image forming apparatus such as a digital copier, the error in copying magnification may fail to fall within a specified range in the manufacture line. After the image forming apparatus is delivered to a user, it may be demanded that the error in the 100%-magnification be eliminated (that is, the erroneous magnification of 100%.+-..alpha. be changed to 100%). In either case the magnification must be adjusted. To this end, the magnification is minutely adjusted in the conventional image forming apparatus.
In an image forming apparatus comprising image forming means that has a polygon mirror for performing optical scanning, an error in the image-forming magnification may occur in two systems, i.e., the reading optical system and the laser optical system, which are shown in FIGS. 10 and 11, respectively.
As seen from FIG. 10, an error in the image-forming magnification may occur in the reading optical system, due to changes in focal distance, which have resulted from inaccurate shapes of the lenses, and due to inaccurate adjusting of the focal distance. (To achieve accurate positioning of lenses, the lens holder must be a high-precision one. The range over which the focal distance can be adjusted is narrow, making it impossible to impart a completely correct focal distance to the reducing optical system.)
As seen from FIG. 11, an error in the image-forming magnification may occur in the laser optical system, due to inaccurate shapes of the lenses, and inaccurate positioning of the lenses.
Image data generated by the reading optical system that has such error-causing factors as described above is supplied to an image processing circuit. The image processing circuit processes the image data, enlarging or reducing the image represented by the data (by altering the method of reading data from a line memory), thereby minutely adjusting the magnification.
When the error in the image-forming magnification is eliminated by enlarging or reducing the image as above, however, the intervals of sampling image data interfere with the intervals of re-sampling the image data for enlarging or reducing the image. Inevitably, a pattern of alternate dark and light stripes, known as "moire," appear in the image. Moire is particularly conspicuous at a magnification nearly equal to 100%. It is generated not only in a high-periodicity image such as a halftone image, but also in a low-periodicity image such as a text image.
To suppress the moire, the image data may be passed through a low-pass filter, thereby smoothing the image represented by the image data. When data representing a text image is passed through a low-pass filter, however, the resultant text image is blurred.